In recent years, it is an increasing trend that offices are networked and documents handled there are digitized and colored. Digitization allows documents to be readily processed and transferred for efficient operations. Colorization allows good-looking, effective documents to be created. As documents are increasingly digitized and colored, multifunction peripherals (MFPs), which are image processing apparatus, are required to effectively capture and output generated image data.
The configuration of an image processing apparatus including an image reader is an element most closely associated with one of original document image capturing performance and image quality of output images in a copy mode. The reader in an image processing apparatus is adapted to include a reduction optical system and a proximity optical system.
The reading resolution of the above optical systems depends on pixel sensors arranged in a primary scan direction. There is a technology called “super-resolution processing” as a process of improving the resolution independent of the number of pixel sensors arranged in a primary scan direction.
While the technology will be described later, the super-resolution processing involves using multiple sets of image data read at the resolution of the sensors provided in the reader to significantly improve the resolution of an output image.
Using the super-resolution processing technology allows image data corresponding to a plurality of frames, for example, read at a resolution of 300 dpi to be converted into image data having a resolution of 1200 dpi.
Processes involved in the super-resolution processing in which high-resolution image data that cannot be obtained by a reader in an image processing apparatus is produced by using multiple sets of image data are described in detail in WO2004/068862.
Japanese Patent Application Laid-Open No. 2006-092450 describes high-resolution processing in which the number of images used as a base of a combined image is controlled according to the image size. This technology involves increasing the number of images to be combined when the image size is small.
To carry out the super-resolution processing described above, however, first of all, it is necessary to prepare successive sets of image data corresponding to a plurality of frames obtained by reading an original document image with the reading position minutely shifted from one to the other with reference to one-frame image data read at the resolution of sensors provided in an image processing apparatus including a reader.
That is, it is necessary to prepare successive sets of image data corresponding to a plurality of frames obtained by slightly shifting the positions of pixels to be read in the primary and/or secondary scan directions from reference image data.
Further, when the image data corresponding to a plurality of frames is obtained, the position at which the original document image is read by a sensor to acquire image data is shifted from the position at which the original document image is read by the adjacent sensor to acquire image data. The shift needs to be smaller than one pixel (sub-pixel) in the primary and/or secondary scan directions.
The greater the resolution of image data generated by the super-resolution processing, the greater the number of necessary image data frames in the image data read at the resolution of the sensors provided in the apparatus.
Performing the super-resolution processing in an image processing apparatus thus allows a low-resolution reader to provide a high-resolution image. To this end, however, it is necessary to satisfy the conditions described above.
In general, however, the reader in a multifunction peripheral, a scanner, and other image processing apparatus uses a line sensor.
That is, the number of frames read in a single reading action is one.
Further, the reader described above reads an original document image by using a group of pixel sensors horizontally arranged in the primary scan direction with the distance between pixels being equal to an integral multiple of the size of a pixel. It is thus disadvantageously not possible to read the original document image by minutely (on a sub-pixel basis) shifting the positions of pixels to be read in the primary scan direction.